The challenges associated with the effective oral delivery of novel and complex but poorly bioavailable (BA) drugs stems primarily from their poor biopharmaceutical and physicochemical characteristics. Very hydrophilic or lipophilic small (less than 500 Da) drugs cannot readily pass through biological membranes or the adjacent aqueous boundary layers. Furthermore, the low efficiency of endocytosis and the restricted substrate specificity of transport proteins further limits the nonpassive absorption of drugs that are larger than 500 Da. Our observation that a 30kDa bioconjugate can be taken up by and pass through intestinal and brain cell monolayers with polymeric drug carriers to overcome normal cell permeability barriers enabling enhanced bioefficacy. Our preliminary in vivo brain uptake data supports this hypothesis. Furthermore, recent preliminary data demonstrates that our Tat-conjugate inhibits HIV in cell culture similar to clinically relevant RTIs. The proposed strategy utilizes a bioconjugate composed of a targeting agent to enhance cell uptake and multiple copies of a drug or drugs linked to a PEG-based polymer. The bioconjugate appears to be absorbed by a novel mechanism that bypasses normal permeability barriers that often limits drug entry into cells. We have identified the mechanistic causes of the poor and variable oral BA of three drugs: saquinavir, UC781, and a Tat inhibitor. Bioconjugates of these three drugs will be prepared. The main objective of the proposed studies is to design, synthesize, characterize and evaluate bioconjugates that will enhance bioefficacy by facilitating transport of anti-AIDS drugs through endothelial, epithelial and target cell barriers. Therefore, the specific aims of the proposed investigations are: (1) To synthesize and characterize a diverse conformational ensemble of placebo bioconjugates by systematically varying the type, number and position of targeting moieties and the molecular weight and branching characteristics of the polymer; (2 ) To establish relationships between bulk property, surface-based, topological and novel composite descriptors of placebo bioconjugates and the transport/uptake kinetics and mechanisms in cultured cells; (3) To design, synthesize and characterize drug bioconjugates using optimal molecular and transport /uptake characteristics of placebo bioconjugates and to characterize the stability, metabolism, and release of drug; (4) To determine oral absorption and brain uptake of drug bioconjugates in model epithelial and endothelial cell culture systems, as well as in well-established animal models. The effectiveness of bioconjugates against HIV-1 infections in cultured cells will also be quantified. The proposed studies will fill important gaps in our understanding of the mechanisms of bioconjugate delivery into and through cells enabling the development of new strategies to overcome biopharmaceutical (e.g., blood-semen or placental) or target cell barriers.